How to remove an item from a StringSet in DynamoDB? - node.js

Question

I have this table in DynamoDB:
{
"lectures": {
"L": [
{
"M": {
"lecture_id": {
"S": "CC0"
},
"students": {
"SS": [
"",
"test"
]
}
}
}
]
}
I want to remove "test" from the StringSet students.
Looking at the docs, I tried this code:
function removeLecture(event){
const students = 'lectures[' + Number.parseInt(event.lecture_id) + '].students';
console.log('test: ' + students);
const params = {
TableName: 'TableName',
Key: {
'Key': key
},
UpdateExpression: `DELETE ${students} :student`,
ExpressionAttributeValues: {
':student' : {
'SS': event.student
}
},
ReturnValues : 'UPDATED_NEW'
}
return ddb.update(params).promise();
}
Anyway, I receive a ValidationException:
START RequestId: 3857a2c2-23a8-4c0c-b080-516669c6e615 Version: $LATEST
2021-08-18T10:21:39.078Z 3857a2c2-23a8-4c0c-b080-516669c6e615 INFO test: lectures[0].students
2021-08-18T10:21:39.831Z 3857a2c2-23a8-4c0c-b080-516669c6e615 INFO ValidationException: Invalid UpdateExpression: Incorrect operand type for operator or function; operator: DELETE, operand type: MAP, typeSet: ALLOWED_FOR_DELETE_OPERAND
I have also tried with ExpressionAttributeValues: {':student' : event.student}, but still I receive the same error with STRING instead of MAP.
How can I remove the String from the StringSet?

Answer 1

After long trying, I discovered the function createSet() of the AWS.DynamoDB.DocumentClient class. I tried to use it, and it works. So the answer is: if you want to remove a String from a StringSet, you have to create another StringSet and remove the latter from the original.
Here is the code:
function removeLecture(event){
const students = 'lectures[' + Number.parseInt(event.lecture_id) + '].students';
console.log('test: ' + students);
const params = {
TableName: 'TableName',
Key: {
'Key': key
},
UpdateExpression: `DELETE ${students} :student`,
ExpressionAttributeValues: {
':student' : ddb.createSet(event.student)
},
ReturnValues : 'UPDATED_NEW'
}
return ddb.update(params).promise();
}